How to connect the guitar (bass) to the iPhone to practice with the iRealBook player?

[dflat]

yes to all.
Ampkit is just one of apps that run in the background. I like Amplitube too (yes it works with Ampkit Link.)

[redstrand]

Export the file as a .wav
Open up Garageband import the track to a vocal track. Make a new track for your guitar or bass. Works like a charm. I do use the iRig but not Amplitube...spent too much money for it on the iPhone and then would have to buy everything again. Garageband works even better.